﻿using System;
using System.Text;
using System.Security.Cryptography;
using System.IO;
using System.Linq;

namespace Moven.Data
{
	public static class StringCipher
	{
		// This constant is used to determine the keysize of the encryption algorithm in bits.
		// We divide this by 8 within the code below to get the equivalent number of bytes.
		const int Keysize = 256;

		// This constant determines the number of iterations for the password bytes generation function.
		const int DerivationIterations = 1000;

		public static string Encrypt(this string plainText, string passPhrase)
		{
			// Salt and IV is randomly generated each time, but is preprended to encrypted cipher text
			// so that the same Salt and IV values can be used when decrypting.  
			var saltStringBytes = Generate256BitsOfRandomEntropy();
			var ivStringBytes = Generate256BitsOfRandomEntropy();
			var plainTextBytes = Encoding.UTF8.GetBytes(plainText);
			using (var password = new Rfc2898DeriveBytes(passPhrase, saltStringBytes, DerivationIterations))
			{
				var keyBytes = password.GetBytes(Keysize / 8);
				using (var symmetricKey = new RijndaelManaged())
				{
					symmetricKey.BlockSize = 256;
					symmetricKey.Mode = CipherMode.CBC;
					symmetricKey.Padding = PaddingMode.PKCS7;
					using (var encryptor = symmetricKey.CreateEncryptor(keyBytes, ivStringBytes))
					{
						using (var memoryStream = new MemoryStream())
						{
							using (var cryptoStream = new CryptoStream(memoryStream, encryptor, CryptoStreamMode.Write))
							{
								cryptoStream.Write(plainTextBytes, 0, plainTextBytes.Length);
								cryptoStream.FlushFinalBlock();
								// Create the final bytes as a concatenation of the random salt bytes, the random iv bytes and the cipher bytes.
								var cipherTextBytes = saltStringBytes;
								cipherTextBytes = cipherTextBytes.Concat(ivStringBytes).ToArray();
								cipherTextBytes = cipherTextBytes.Concat(memoryStream.ToArray()).ToArray();
								memoryStream.Close();
								cryptoStream.Close();
								return Convert.ToBase64String(cipherTextBytes);
							}
						}
					}
				}
			}
		}

		public static string Decrypt(this string cipherText, string passPhrase)
		{
			// Get the complete stream of bytes that represent:
			// [32 bytes of Salt] + [32 bytes of IV] + [n bytes of CipherText]
			var cipherTextBytesWithSaltAndIv = Convert.FromBase64String(cipherText);
			// Get the saltbytes by extracting the first 32 bytes from the supplied cipherText bytes.
			var saltStringBytes = cipherTextBytesWithSaltAndIv.Take(Keysize / 8).ToArray();
			// Get the IV bytes by extracting the next 32 bytes from the supplied cipherText bytes.
			var ivStringBytes = cipherTextBytesWithSaltAndIv.Skip(Keysize / 8).Take(Keysize / 8).ToArray();
			// Get the actual cipher text bytes by removing the first 64 bytes from the cipherText string.
			var cipherTextBytes = cipherTextBytesWithSaltAndIv.Skip((Keysize / 8) * 2).Take(cipherTextBytesWithSaltAndIv.Length - ((Keysize / 8) * 2)).ToArray();

			using (var password = new Rfc2898DeriveBytes(passPhrase, saltStringBytes, DerivationIterations))
			{
				var keyBytes = password.GetBytes(Keysize / 8);
				using (var symmetricKey = new RijndaelManaged())
				{
					symmetricKey.BlockSize = 256;
					symmetricKey.Mode = CipherMode.CBC;
					symmetricKey.Padding = PaddingMode.PKCS7;
					using (var decryptor = symmetricKey.CreateDecryptor(keyBytes, ivStringBytes))
					{
						using (var memoryStream = new MemoryStream(cipherTextBytes))
						{
							using (var cryptoStream = new CryptoStream(memoryStream, decryptor, CryptoStreamMode.Read))
							{
								var plainTextBytes = new byte[cipherTextBytes.Length];
								var decryptedByteCount = cryptoStream.Read(plainTextBytes, 0, plainTextBytes.Length);
								memoryStream.Close();
								cryptoStream.Close();
								return Encoding.UTF8.GetString(plainTextBytes, 0, decryptedByteCount);
							}
						}
					}
				}
			}
		}

		static byte[] Generate256BitsOfRandomEntropy()
		{
			var randomBytes = new byte[32]; // 32 Bytes will give us 256 bits.
			using (var rngCsp = new RNGCryptoServiceProvider())
			{
				// Fill the array with cryptographically secure random bytes.
				rngCsp.GetBytes(randomBytes);
			}
			return randomBytes;
		}
	}
}